Rendering low poly model to 2D, make facets stand out more
You have to create contrast in your lighting to bring out the facets. Lighting from the exact same direction of the camera does the opposite, it makes everything flat.

This image I created by starting with your blend file and then:


Removing the Front Light object
Removing the Front Light material from the Top/Bottom Light objects
Increasing the emission strength
Moving the light objects towards/away from the mesh object until the lighting is nice




By adding a bit of glossyness to the mesh material we can bring out the facets even more:



This is the material node tree: